1. Field of the Invention
The present invention relates to the preparation of tertiary alkyl ethers. In particular, the invention concerns a novel process for preparing such ethers, wherein isoolefms, in particular C.sub.4 -C.sub.7 isoolefins of an olefinic hydrocarbon feedstock are reacted with a suitable alkanol to prepare the corresponding ethers. These ethers are recovered and, if necessary, further processed in order to prepare, e.g, a motor fuel component or a petrochemical feedstock. A hydrocarbon stream of unreacted hydrocarbons emanating from the etherification reaction is subjected to alkanol recovery and oxygenate-removal enabling direct use of hydrocarbon effluent for another process, such as alkylation.
2. Description of Related Art
Tertiary alkyl ethers improve the anti-knocking characteristics of motor fuels and reduce the concentration of detrimental components in the exhaust gases. Of the alkyl tert-alkyl ethers used as components of motor fuels, the following can be mentioned: methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), t-amyl methyl ether (TAME), t-amyl ethyl ether (TAEE) and t-hexyl methyl ether (THME). These ethers are produced by etherification of an isoolefm with a monovalent aliphatic alcohol (in the following also called an "alkanol"). The isoolefin feedstock can comprise pure C.sub.4 or C.sub.5 components or a mixed hydrocarbon feedstock containing a range of C.sub.3 -C.sub.7 and heavier hydrocarbons. Typically ion exchange resins are used as etherification catalysts.
In the conventional etherification processes, ethers are produced in a series of reactors to which a mixture of isoolefrns and an alkanol are fed. The ether product is separated from the reaction mixture by ordinary distillation. The bottom product of the distillation process comprises the ether(s) and, depending on the process, also some of the unreacted heavier hydrocarbons. The overhead stream consists of an unreacted alkanol together with some lighter hydrocarbons, dialkyl ethers and, possibly, water.
Before the unreacted hydrocarbons in the overhead stream can be used for chemical processing by, say, alkylation, it has to be freed from oxygenates and other contaminants. Thus, the concentration of alkanols and other oxygenated hydrocarbons, such as dialkyl ethers, has to be reduced to below 100 ppm, preferably below 50 ppm. According to conventional technology, this separation and decontamination operation requires rather complex process steps. To take MTBE production as an example, it can be noted that the overhead from the reaction product distillation, containing methanol-rich C.sub.4 's, is first washed in a countercurrent liquid-liquid extraction column with water to form a methanol-water solution. This solution is recovered from the bottom of the extraction column and fed into a fractionation tower, wherein methanol is recovered. The C.sub.4 raffinate from the extraction column is then fed into another fractionation tower, called the oxygenate removal unit, in order to separate the light oxygenates such as dialkyl ethers, trace alkanol and water from the C.sub.4 raffinate. Thus, one extraction column and two distillation columns are required to recover the methanol and to produce an essentially oxygenate-free C.sub.4 raffinate. Moreover, the purification of the unreacted hydrocarbons in the overhead stream typically requires the use of an oxygenated compound, water, which just after introduction must be carefully removed from the raffinate.
Alternatively, a specific molecular sieve system may be employed for oxygenates removal.
It is known in the art of TAME production (PCT Patent Application No. WO 93/1903) that the amount of alkanol in the overhead stream can be minimized by operating the fractioning in such a way that most, and preferably practically (nearly) all, of the alkanol in the distillate is bound to C.sub.4 hydrocarbons, forming an azeotrope with them. Such a product can be blended with fuels often without any additional purification steps. However, because the amount of alkanols in the overhead stream depends on the amount of unreacted C.sub.4 's of the reaction mixture, the overhead stream obtained from etherification of an isobutylene feed according to the above art will contain too much alkanol for most of the downstream processes, in particular alkylation.